The use of wireless communication between different parts of a device and/or different devices has many advantages. Wireless communication simplifies installation. It eliminates the need to layout cables or wires between the devices, as well as identify and hook up to specific connection sockets. It allows greater freedom in positioning of the device, and the use of mobile or hand held devices. Some examples of such devices are a remote control for a TV (or other device), a computer and its input and/or output peripheral devices (e.g. mouse, keyboard, screen, printer) and an audio source device and its respective surround speakers.
Commonly, wireless devices use RF (radio frequency) or IR (Infrared) technology. In some characteristics RF has advantages over IR, and in some characteristics IR has advantages over RF. Typically, RF is able to support longer range transmissions and transmissions through walls and other opaque objects. This is useful for devices such as a wireless (cordless) telephone, which can be used throughout a house, or for wireless computer networks. In contrast IR is generally limited to a single room or enclosure. IR transmissions are reflected and scattered by various objects and surfaces. IR transmissions penetrate glass but do not penetrate walls. Typically, IR suffers less interference than RF, as it uses an optical carrier for transmission over the wireless medium. It especially almost has zero interference from devices outside the room or enclosure it operates in. IR is more secure since it is less susceptible to eaves dropping from outside the enclosure. These characteristics make it ideal for devices that function in a single room/enclosure, for example remote controls, input and output peripheral devices of a computer, wireless speakers for home theater systems as well as wireless video systems (like digital TV).
In applying IR wireless communication, there could be a few types of implementations. There are implementations wherein the transceivers (transmitter & receiver) need to be aimed one towards the other (referred to as direct IR), and there are implementations wherein the transceivers do not need to be aimed one towards the other (referred to as non-direct IR). There are implementations that require keeping a non-blocked line of site (LOS) between the communicating transceivers, and there are implementations, which do not require a non-blocked line of site (LOS) between the communicating transceivers (although they might require to be directed toward the other transceivers). A connection which is simultaneously non-direct and non-LOS is referred to as diffused connection. A diffused connection is the most flexible, since it allows a relatively loose deployment of the transceivers around a room or an enclosure. On the other hand a diffused connection typically requires larger power emission from the transmitter part of the transceiver, as the diffused infrared signal suffers greater losses than direct and line of sight wireless optical communication systems.
It should be noted that although IR transmissions are generally free from commonly known RF interferences, they might still be affected by natural and artificial ambient light sources such as sunlight, plasma TV emissions and electronic ballast florescent lamps. In a base-band wireless optical communication system, communication is usually governed by sending short pulses (that mimic ON (‘1’) and OFF (‘0’) ‘bit’ values) over the wireless medium. Typically, such ambient light sources, through optical and electronic noise interference mechanisms, can cause a ‘1 (ON) bit value to be shifted over by one fall bit position to an adjacent bit position (either left or right) causing a ‘1’ (ON) bit value to be detected as a ‘0’ (OFF) bit value, and an adjacent ‘0’ (OFF) bit value to be detected as a ‘1’ (ON) bit value (e.g. two errors). This can be referred to as a full bit position jitter. Similar phenomenon can occur in multipath propagation infrared systems, especially in diffused connections or channels, which suffer from multiple diffuse reflections. This phenomenon is usually known as inter symbol interference, or ISI. Additionally, IR signal strength over the detector plane in the receiver part of the transceiver is dependent on the distance and geometry of the optical path (e.g. direct, reflected) between the communicating transceivers. The larger the distance and/or the number of bounces the optical signal needs to traverse between the transmitter and receiver, the more susceptible the link is to errors, as signal strength typically degrades as the square of the distance. As an example, a ‘1’ (ON) bit value may arrive so weak at the receiver it will erroneously be detected as a ‘0’ (OFF) bit value by that receiver's detection circuitry. This is referred to as pulse erasure. Similarly, a ‘0’ (OFF) bit value may be affected in such a way by added noise and interference causing it to erroneously be detected as a ‘1’ (ON) bit value. This is referred to as a foreign pulse.
Typically, direct transmission of the raw communication data as binary bits (pulses) like in simple on-off keying (OOK) base-band modulation is problematic, since various reception circuits (e.g. a high pass filter that is used in the receiver device to filter out low frequency noise) tend to have difficulty in dealing with long consecutive sequences of ‘1’ (ON) or ‘0’ (OFF) bit values. In order to overcome this problem, it is common practice to encode the raw binary data using various, more sophisticated, base-band modulation techniques, for example: Manchester modulation, L-Ary pulse position modulation (LPPM) or differential PPM (DPPM) as well as run length limited (RLL) modulation techniques. In these techniques, for any incoming raw communication data, the length of consecutive ‘1’ (ON) and ‘0’ (OFF) bit values has a certain predetermined value.
Various types of PPM, and related or similar methods, are referred to as a base-band modulation techniques since the raw data bits are converted or mapped directly into another modulated signal with an ON pulse representing a ‘1’ bit value, and the lack of an ON pulse representing a ‘0’ bit value. In base-band modulation techniques the raw data bits are not modulated on a high frequency based carrier as is typically performed in RF wireless systems. PPM is an orthogonal base-band modulation technique that offers a decrease in average power requirement compared to OOK, at the expense of an increased bandwidth requirement. In PPM, a fixed number of bits N, termed as a symbol, with 2N (2 to the power of N) possible values, are encoded by dividing the time duration of the N bit symbol to 2N time positions, referred to as chips, and transmitting a pulse (e.g., a ‘1’ value chip) in one of the time positions (chips) of the signal representing the original N bit symbol. As an example, a 2-bit symbol with 4 possible values (e.g. ‘00’, ‘01’, ‘10’ and ‘11’) is represented by 4 half-bit time positions (chips), wherein each position directly represents one of the 4 possible symbol values. Likewise 4-bit symbols with 24=16 possible values (e.g. ‘0000’, ‘0001’, ‘0010’, . . . , ‘1111’) are represented by 16 quarter-bit time positions (chips), wherein each position directly represents one of the 16 possible symbol values.
The use of PPM modulation results in a single short pulse (‘1’ chip value) within the overall time duration of any possible value of the N bit symbol. For example, a 4-bit symbol ‘0000’ is represented by a single short pulse at the first position of the train of the 16 possible chip positions, and ‘1111’ is represented by a single short pulse in the last position of the train of 16 possible chip positions. The converted representation from raw data symbols to base-band modulated chips is generally referred to as a codeword representing the original set of bits (or symbol).
A PPM modulated codeword has the same time duration as the original raw data symbol, however the energy required for transmission over the wireless medium is typically reduced since all possible symbol values are represented by a single short pulse (chip), for example ‘1111’ is represented by a short pulse (‘1’), which is a sixteenth of the size (in time and energy) of the original raw data symbol (‘1111’). Additionally, each symbol, after mapping to a PPM codeword comprises a single pulse (chip), which is easier to handle by the receiver device, in contrast to the original bit representation which can have no pulse (‘0000’) or varying length pulses (e.g. ‘1100’) according to the raw data bit values.
In systems where the transmissions are transmitted to multiple receivers, and/or are needed for taking immediate action, like in real time streaming media devices (e.g. feeding the next device in the track with the streaming type communication), it is generally not feasible to implement a simple system to request retransmission if transmissions are received erroneously. Typically, error detection methods can be used to recognize that a transmission has an error, for example by transmitting a CRC or checksum field, which is used to authenticate the transmitted data, and if a discrepancy is detected it is evidence of the existence of an error. In more advanced techniques, referred to as forward error correction (FEC) methods, extra redundant data is transmitted (e.g. typically a set of parity bits) to allow detection of some errors and correction of part of these errors. Typically, the extent of redundancy to be used in the FEC technique depends on the characteristics of the wireless transmission channel, the specific modulation technique used, and the acceptable specified wireless system bit error rate (BER) over the wireless medium.
Generally, when transmitting base-band (e.g. pulsed) infrared wireless transmissions, the greater the distance between the communicating transceivers, or the noisier the environment (e.g. direct sunlight, artificial light sources), the greater the number of errors that are manifested within the originally sent wireless signal. Typically, overcoming transmission errors (up to a certain extent) requires retransmission of data, when an error occurs, or transmission of extra redundancy bits to facilitate a forward error correction scheme. Retransmission of data from the transmitter severely degrades the effective bandwidth of the wireless link, and limits the implementation feasibility of wireless infrared systems, especially for streaming type of systems like wireless audio and video systems.